nth_element 使用小例

最新推荐文章于 2024-05-30 21:13:03 发布

chenglai5522

最新推荐文章于 2024-05-30 21:13:03 发布

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原文链接：https://my.oschina.net/imtoby/blog/71330

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本文展示了一个使用C++标准库函数nth_element的例子，该函数用于部分排序容器中的元素，以便第n个元素位于其应该在完全排序容器中的位置。示例中包括使用greater<int>()进行降序排列和使用自定义比较函数MoreThan进行排列。

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#include <vector>
#include <algorithm>
#include <functional> // For greater<int>( )
#include <iostream>
using namespace std;
// Return whether first element is greater than the second
struct MoreThan: public binary_function<int,int,bool> {
bool operator()(const int& a1,const int& a2) const {return a1 > a2;}
};

// bool UDgreater (const int elem1,const int elem2 )
// {
// return elem1 > elem2;
// }

int main( )
{
using namespace std;
vector <int> v1;
vector <int>::iterator Iter1;

int i;
for ( i = 0 ; i <= 5 ; i++ )
{
v1.push_back( 3 * i );
}

int ii;
for ( ii = 0 ; ii <= 5 ; ii++ )
{
v1.push_back( 3 * ii + 1 );
}

int iii;
for ( iii = 0 ; iii <= 5 ; iii++ )
{
v1.push_back( 3 * iii +2 );
}

cout << "Original vector:\n v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")" << endl;

nth_element(v1.begin( ), v1.begin( ) + 3, v1.end( ) );
cout << "Position 3 partitioned vector:\n v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")" << endl;
cout << "Position 3 Value is " << v1.at(3) <<endl;

// To sort in descending order, specify binary predicate
nth_element( v1.begin( ), v1.begin( ) + 4, v1.end( ),
greater<int>( ) );
cout << "Position 4 partitioned (greater) vector:\n v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")" << endl;
cout << "Position 4 Value is " << v1.at(4) <<endl;

random_shuffle( v1.begin( ), v1.end( ) );
cout << "Shuffled vector:\n v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")" << endl;

// A user-defined (UD) binary predicate can also be used
nth_element( v1.begin( ), v1.begin( ) + v1.size()/2, v1.end( ), MoreThan());
cout << "Position 5 partitioned (UDgreater) vector:\n v1 = ( " ;
for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )
cout << *Iter1 << " ";
cout << ")" << endl;

cout << "mid Position Value is " << v1.at(v1.size()/2) <<endl;

system("pause");
return 0;
}

转载于:https://my.oschina.net/imtoby/blog/71330